Rounding up the costs and benefits of herbicide use.

I n recent years, we have seen an exponential increase in the use of herbicides, such as RoundUp, that contain the active ingredient glyphosate (1). Glyphosate kills plants by inhibiting the production of aromatic amino acids in the shikimate pathway (2). This herbicide is popular because it works well and has been shown to be relatively ecologically ‘‘safe’’; it has low toxicity to animals, including humans, and degrades rapidly upon contact with soil (3). Taking advantage of its efficacy and relative safety, biotechnology companies have been genetically engineering crops for resistance to glyphosate, which allows farmers to weed their fields by spraying herbicides that inhibit the weeds but not the crop. Although glyphosate is less ecologically damaging than some other herbicides, it may nonetheless come back to haunt us through the evolution of its target weeds. In a recent issue of PNAS, Baucom and Mauricio (1) showed that not only can herbicide use lead to evolution of resistance but also it results in the evolution of tolerance, the ability to compensate for damage. Importantly, they also suggested that there may be ways to halt or slow this evolutionary process because of the costs to evolving tolerance. Human-caused evolution is becoming a dominant factor in biology (4). Not only do humans influence the ecology of every ecosystem, but also we change the survival and reproduction of organisms, leading to evolution (genetic change) in the survivors. For example, our preference for harvesting larger animals has imposed selection for reproduction at smaller sizes and has resulted in the evolution of smaller individuals in populations of game animals (5) and fish (6). Another way humans cause rapid evolutionary change is through the use of chemicals designed to kill other organisms (e.g., antibiotics, pesticides, herbicides). These compounds cause strong selection; when applied to populations of microbes, insects, or plants, most individuals die. However, any individual with a trait that allows them to survive and reproduce, despite the deadly onslaught, will pass on their genes to the next generation. Over time, survivors will increase in number, changing (evolving) the genetic composition of the population. There are two ways organisms cope with damage: they can resist receiving it or tolerate it. Most of us are familiar with resistance, which occurs when traits are present that stop or decrease the activity of the pesticide (or pathogen or herbivore), allowing the organism to be relatively unaffected. Much less well understood are tolerance traits, which do not reduce or eliminate damage but instead allow the organism to survive and reproduce despite being damaged (Fig. 1). It was a breakthrough for ecologists when we finally realized that resistance